1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus for exposing photosensitive substrates, such as silicon plates and glass, to light through patterns designed for devices, such as semiconductors including an IC, an LSI, etc., a liquid crystal panel, a magnetic head, a CCD (image sensor), and so on.
2. Description of the Related Art
In manufacturing an IC, an LSI, a liquid crystal element, etc., by photolithography, a projection exposure apparatus (projection aligner) is employed. The projection exposure apparatus is arranged to perform an exposure by projecting through a projection optical system a pattern of a photomask or a reticle (hereinafter referred to as a xe2x80x9cmaskxe2x80x9d) onto a substrate, such as a silicon plate or a glass plate, which is coated with a photoresist or the like (hereinafter referred to as a xe2x80x9cwaferxe2x80x9d in general).
FIG. 1 schematically illustrates the arrangement of a conventional projection exposure apparatus. In FIG. 1, there are illustrated a KrF excimer laser 191 used as a light source, an illumination optical system 192, illumination light 193, a mask 194, exposure light 195 on the object side, a projection optical system 196, exposure light 197 on the image side, a photosensitive substrate (wafer) 198, and a substrate stage 199 which holds the photosensitive substrate 198.
In the conventional projection exposure apparatus, a laser beam emitted from the excimer laser 191 is led to the illumination optical system 192. At the illumination optical system 192, the laser beam is converted into the illumination light 193 having a light intensity distribution, a luminous distribution, etc., which are predetermined. The illumination light 193 falls on the mask 194. A circuit pattern which is to be eventually formed on the photosensitive substrate 198 is beforehand formed on the mask 194 with chromium or the like. The incident illumination light 193 passes through the mask 194 and is diffracted by the circuit pattern to become the object-side exposure light 195. The projection optical system 196 converts the exposure light 195 into the image-side exposure light 197 to image the circuit pattern on the photosensitive substrate 198 at a predetermined magnification with sufficiently small aberrations. As shown in an enlarged view at the lower part of FIG. 1, the image-side exposure light 197 converges on the photosensitive substrate 198 at a predetermined NA (numerical aperture=sin xcex8) to form the image there. To have the circuit pattern formed in a plurality of shot areas on the photosensitive substrate 198, the substrate stage 199 is arranged to be movable stepwise to vary the relative positions of the photosensitive substrate 198 and the projection optical system 196.
However, with the conventional projection exposure apparatus using the KrF excimer laser arranged as described above, it is difficult to form a pattern image of a line width not greater than 0.15 xcexcm.
The reason for this difficulty is as follows. The resolution of the projection optical system is limited by a trade-off between an optical resolution and the depth of focus due to the wavelength of the exposure light. The resolution R of the resolving pattern of the projection exposure apparatus and the depth of focus DOF can be expressed by the following Rayleigh""s formulas (1) and (2):                     R        =                  k1          ⁢                      λ            NA                                              (        1        )                                DOF        =                  k2          ⁢                      λ                          NA              2                                                          (        2        )            
In the above formulas, xcex represents the wavelength of the exposure light, NA represents a numerical aperture indicative of the brightness of the optical system on the light exit side, and k1 and k2 represent constants which are normally between 0.5 and 0.7.
According to the formulas (1) and (2), in order to make the resolution R smaller for a higher degree of resolution, it is necessary either to make the wavelength xcex smaller for a shorter wavelength or to make the value NA larger for a higher degree of brightness. At the same time, however, the depth of focus DOF required for a necessary performance of the projection optical system must be kept at least at a certain value. This requirement imposes some limitation on the increase of the brightness value NA.
There is another known exposure method which does not depend on the formulas (1) and (2). FIG. 2 is a schematic diagram for explaining such an exposure method. Referring to FIG. 2, a coherent light beam emitted from a laser beam source 151 is divided by a half-mirror 152 into two light fluxes. Mirrors 153a and 153b are arranged to deflect the two light fluxes respectively at some angles to cause the two light fluxes to join together on a photosensitive substrate 154 in such a way as to form interference fringes there. The photosensitive part of the photosensitive substrate 154 is allowed to sense light according to a distribution of light intensity made by the interference fringes. Then, a periodic protrusion-and-recess pattern is formed according to the distribution of the light intensity by a developing process.
The resolution R obtained by the above exposure method is expressed by the following formula (3), wherein the resolution R is assumed to be the width of each of lines and spaces, i.e., the width of each of the bright and dark parts of the interference fringes, xcex8 represents the angle of incidence on the substrate 154 of the two light fluxes 151a and 151b, and NA=sin xcex8.                                                         R              =                              λ                                  4                  ⁢                  sin                  ⁢                                      xe2x80x83                                    ⁢                  θ                                                                                                        =                              λ                                  4                  ⁢                  NA                                                                                                        =                              0.25                ⁢                                  λ                  NA                                                                                        (        3        )            
As is understandable by comparing the formulas (3) and (1) with each other, the constant k1 becomes 0.25 (k1=0.25) according to the exposure method shown in FIG. 2. Considering that the value of the constant k1 of the conventional projection exposure method is between 0.5 and 0.7, the resolution obtainable by the exposure method shown in FIG. 2 is more than two times as high as the resolution obtainable by the conventional exposure method. According to the exposure method shown in FIG. 2, assuming that xcex is 0.248 xcexcm and NA is 0.6, for example, the resolution R becomes 0.10 xcexcm.
However, the exposure method shown in FIG. 2 presents a serious problem in that a circuit pattern composed of diverse shapes like semiconductor element patterns hardly can be obtained by carrying out an exposure according to that method, because only such a line-and-space pattern that has a uniform pitch over its whole area is obtainable according to the method of making an exposure through the interference of light fluxes as shown in FIG. 2.
This problem can be solved, for example, by a known multiple exposure method whereby the projection exposure by the method of FIG. 1 and the two-light-flux interference exposure by the method of FIG. 2 are alternately made in combination for one and the same area of a photosensitive substrate one after another without carrying out any developing process on the substrate at intervals between these exposures.
However, the two-light-flux interference exposure in the conventional multiple exposure method necessitates either the use of the apparatus shown in FIG. 2, in addition to the projection exposure apparatus, or the use of a special mask such as a phase-shifting mask in using the projection exposure apparatus.
It is an object of the invention to provide a projection exposure apparatus arranged to permit a multiple exposure to be simply carried out.
To attain the above object, in accordance with an aspect of the invention, there is provided a projection exposure apparatus having a multiple exposure mode, which comprises an illumination system and a projection system, the projection system including means for automatically or manually supplying into an optical path a filter which blocks a zero-order light beam among a plurality of light beams coming from a mask illuminated by the illumination system, wherein an exposure step in the multiple exposure mode is performed in a state in which the filter has been supplied into the optical path.
In the projection exposure apparatus according to the above-stated aspect, the filter is arranged to be supplied to a position of a pupil of the projection system or to a neighborhood of the position of the pupil. The mask which is used when the filter is supplied to the position of the pupil of the projection system or to the neighborhood of the position of the pupil has a periodic pattern of a pitch which is two times a value obtained by dividing a pitch (P) of a periodic pattern image to be formed on an image plane by a projection magnification (M) of the projection system.
Further, the exposure step in the multiple exposure mode is performed in such a manner that a first exposure pattern having an exposure amount not exceeding a threshold value of an object to be exposed is formed, an exposure stage different from the exposure step in the multiple exposure mode is performed in such a manner that a second exposure pattern having an exposure amount exceeding the threshold value and an exposure amount not exceeding the threshold value is formed, and the respective exposure amounts are determined in such a manner that a composite exposure pattern formed by combining the first and second exposure patterns is in such a relation to the threshold value that a desired circuit pattern is formed.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.